1. Field of the Invention
The present invention relates to high-speed bonding tools of the type used in automatic wire bonders to make fine wire interconnections between the pads on a semiconductor chip and a carrier or lead frame.
2. Description of the Prior Art
Capillary wire bonding tools are known in the prior art and are classified in International Class B23K 3/00 which corresponds to U.S. Class 219, Subclass 85D and 85F. Bonding wedges are cross-referenced with bonding capillaries and are classified in International Class B23K 1/06 as well as in U.S. Class 228, Subclasses 1 to 5.
Standard high-speed bonding capillaries were developed for use on automatic gold ball thermosonic wire bonders such as the bonder shown and described in U.S. Pat. No. 4,266,710. The working face on the tip of the bonding capillary used in such wire bonders is provided with a conical recess or inner chamfer for making a first ball bond which comprises bonding a spherical ball on the end of a fine wire. The working face of the tip of the prior art bonding capillary is usually provided with an annular ring having a face angle up to 15 degrees for making the second or wedge bond which comprises mashing, bonding and severing the fine interconnecting wire on the second bond target. The annular ring working face on such standard capillaries permitted the bonding tool to move from a first bond position in any direction to a second bond position and then wedge bond the fine wire to a lead frame or carrier at second bond.
In contrast thereto, automatic thermosonic wedge bonders such as the bonder shown in U.S. Pat. No. 4,239,144 were provided with a rotary heads which permitted rotational alignment of the working face of the wedge bonding tool along a directional axis between the first and the second bond target. The rotary head feature permitted the tip of the working face of the bonding wedge bonding tool to be made more narrow than conventional capillary bonding tools. Because of the ability of the rotary bonding head wedge bonding machine to employ narrow face wedges, it was generally assumed that rotary head automatic wedge bonding machines have the ability to make fine wire interconnections on closer pitch spacings than bonding machines which employed ball bonding capillaries having an annular ring working face for making the second bond. One of the reasons that wedge bonding tools are capable of making first bonds on finer and closer pitch centers than the capillary ball bonding tools is that the wedge bonding tool does not create a mashed ball at the first bond, but instead makes a mashed wire bond substantially identical to a second wedge bond. Such mashed wire bonds have less shear strength than a first ball bond. Heretofore, wedge wire bonds made at first bond did not provide bonds as consistently strong in tension as ball bonds.
The continuing miniaturization of semiconductor chips is driving the pad size toward 3.3 mils squares placed on 3.8 mils centers. The recommended ball size for 1 mil gold wire is about 2.4 mils which when mashed and bonded at first bond becomes about 3.5 mils or 3.5 times the wire diameter for 1 mil wire. For prior art ball bonders, the pad size is limited by the mashed and bonded ball size which also limits the pitch between the centers of the pads.
It would be desirable to provide a capillary or wedge bonding tool which could be used on existing type automatic wire bonders that would preserve the advantage of ball bonding capillaries and combine them with the advantages of wedge bonding tools used on rotary head automatic wedge bonding machines.